A U.S. intelligence bulletin warned today that the violent outrage aimed at U.S. embassies spawned by a movie mocking the Prophet Mohammed could be spread to America by extremist groups eager to "exploit anger."

The Joint Intelligence Bulletin issued by the FBI and the Department of Homeland Security said "the risk of violence could increase both at home and abroad as the film continues to gain attention."

"First responders should remain aware of the potential for spontaneous large crowds and protests that could overwhelm resources and should be vigilant for possible efforts to encourage peaceful protesters to commit acts of violence," the warning said.

It urged "faith-based organizations to promptly report suspicious activities that could indicate pre-operational plotting against Jewish, Coptic, Islamic, or any other faith-based communities."

It added, however, that there was no information regarding any specific threats.

The bulletin warned that "violent extremist groups in the United States could exploit anger over the film to advance their recruitment efforts."

It goes on to note that angry protests in Benghazi, where four Americans were killed, and Cairo "mirror past incidents prompted by events perceived as anti-Islamic, which spurred sudden violence against U.S. interests overseas."

The bulletin was issued as fresh violence erupted at U.S. embassies in Yemen and Egypt, and the State Department said that 50 overseas missions have been given additional security and warned U.S. citizens in their area to be "extra vigilant." Some embassies warned of protests on Friday, the Muslim sabbath.

President Obama called the presidents of Egypt and Libya where he thanked them for their help but reminded them of the importance of protecting American embassies and consulates. And in Afghanistan, President Hamid Karzai canceled an overseas trip because of brewing anger over the film, "Innocence of Muslims," which ridicules Islam's founder the Prophet Mohammed.

Two U.S. Navy missile destroyers, the USS Laboon and USS McFaul, were moved near the coast of Libya as an extra precaution.

Libyan officials said several people have been arrested for the attack on a U.S. consulate in Benghazi that killed Ambassador Christopher Stevens, Information Management Officer Sean Smith, security personnel and former Navy SEALs Tyrone S. Woods and Glen A. Doherty but U.S. officials have not confirmed that.

Wanis el-Sharef, eastern Libya's deputy interior minister, told the Associated Press the attacks were suspected to have been timed to mark the 9/11 anniversary and that the militants used civilians protesting the movie as cover for their action.

Western intelligence sources told ABC News that it appears the movie was the "catalyst," but it would not be surprising if a militant group "would seek to take advantage for an opportunity to strike."

FBI agents from New York were being dispatched to Libya to assist in the investigation.

The worst violence today erupted in Sana'a, Yemen, where thousands of angry protesters circled the U.S. embassy and breached its outer wall, but were prevented from entering the embassy compound.

Yemeni forces threw tear gas as protesters were seen scrambling over fences and the main gate, firing gunshots as they tried to stop the demonstrators.

Protesters in Sana'a removed the embassy's sign on the outer wall and set tires ablaze, The Associated Press reported. Once inside the compound, they took down and burned the U.S. flag. Security guards at the embassy fired warning shots to stop them.

Yemen Protesters Attack U.S. Embassy

According to a U.S. Embassy spokesman in Yemen, all personnel are safe. A senior official in the Obama administration said that the Yemeni government had aided the U.S. in maintaining order.

Abdrabuh Mansour Hadi, president of the Republic of Yemen, apologized to the U.S. for the attacks, the Yemeni embassy in Washington, D.C. said in a statement released early Thursday.

Protests outside the U.S. Embassy in Cairo turned violent again Wednesday night and early this morning. Security forces used tear gas and warning shots to fight off the protesters, who managed to break through a barbed wire barrier. The efforts of security forces managed to push demonstrators back more than 600 feet to Tahrir Square.

Mahmoud Hussein, the secretary general of the Muslim Brotherhood in Cairo, has called for peaceful protests after Friday prayers in front of Mosques in all cities across Egypt "in response to the insults to the religious beliefs and the Prophet."

In Berlin, the U.S. Embassy said early Thursday that its consulate in Berlin has been evacuated as a precaution after an employee reported a strange smell from an envelope, but German police said later it was a false alarm.

Secretary of State Hillary Clinton tried today to deflect some of the anger away from the U.S.

"Let me state very clearly -- and I hope that is obvious -- that the United States government had absolutely nothing to do with this video," she said. "We certainly hope and expect that there will be steps taken to avoid violence and prevent the escalation of protests into violence."

She said the State Department is monitoring protests in Yemen and elsewhere. Clinton said that she believes the film is "disgusting and reprehensible," but that there is no justification for responding to it with violence.

Just who made the movie is a mystery. Nakoula Basseley Nakoula, 55, told The Associated Press in an interview near Los Angeles that he was a manager of the company that produced "Innocence of Muslims." Nakoula denied that he'd directed the film, and said he knew the self-described filmmaker, using a pseudonym name identified only as "Sam Bacile." But the cell phone number The AP used Tuesday to contact the filmmaker was traced to the same Los Angeles area address where The AP found Nakoula.